1. Field of the Invention
This invention relates to a satellite broadcasting tuner which can cope with high-definition television satellite broadcasting which is broadcast in the MUSE (Multiple Sub-Nyquist Sampling Encoding) system.
2. Description of the Prior Art
Reception of satellite broadcasting which is broadcast making use of a communications satellite requires a down converter for converting a frequency of radio waves of satellite broadcasting received, for example, by way of a parabola antenna, and a satellite broadcasting tuner for selecting a desired broadcasting channel from within an output of the down converter and demodulating a video signal of the selected broadcasting channel. Such satellite broadcasting tuner receives a signal having a frequency of 1 GHz obtained by down-conversion by a down converter and frequency converts the received signal to form an intermediate frequency in which video signals of several channels are included. Such satellite broadcasting tuner has an AFT (automatic fine tuning) function wherein a local oscillation frequency supplied to a frequency converter is controlled in accordance with a level of a demodulated video signal so that an appropriate intermediate frequency may be obtained automatically (average value AFT).
By the way, when a high-definition television signal which provides a video signal of a high definition is transmitted using a satellite broadcasting channel, the signal band of such high-definition television signal is wider than that of ordinary satellite broadcasting. For example, if a high-definition television signal is transmitted in accordance with the MUSE system wherein the base band is compressed to 8:1 MH, then a satellite broadcasting tuner must necessarily have at least a reception bandwidth of 27 MH or so. Consequently, if a selected channel frequency of a satellite broadcasting channel in which a high-definition television signal is broadcast is controlled in accordance with such average AFT as described above, then it is difficult to cause a central value of a frequency band of the desired satellite broadcasting channel to coincide with a central frequency of an intermediate frequency of a satellite broadcasting tuner, and consequently, the quality of a video signal is deteriorated.
Thus, the applicant of the present patent application has provided a satellite broadcasting tuner with a channel selecting apparatus wherein, when a satellite broadcasting channel in which a high-definition television signal is broadcast is selected, frame synchronizing data included in an audio signal constituting such high-definition television signal are detected, and then when such frame synchronizing data are detected, an average value AFT operation is stopped and a channel selecting frequency is fixed for reception of the channel.
With a tuner of such channel selecting frequency selecting system, since a frame synchronizing data signal is detected from within a received signal to detect that a channel being received at present is a high-definition television broadcasting channel and then a channel selecting frequency is fixed so that a central value of an intermediate frequency which is formed by the channel selecting frequency may coincide with a central value of a signal band frequency of the MUSE system, high-definition television broadcasting can be received in a high quality. However, the tuner has a problem that, if the frequency of a converter for lowering the frequency of radio waves received, for example, by way of a parabola antenna varies, then the central frequency varies following such variation.